Amphibious. The dragon can breathe air and water.

Colossal. The dragon is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

In addition, whenever the dragon makes a weapon attack, regardless of whether the attack hits or misses, each creature within 10 feet of the attack's target other than the target or the dragon itself must succeed on a DC 25 Dexterity saving throw or takes half the attack's damage.

Empowered Attacks. The dragon's weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks.

Forest Strider. The dragon ignores difficult terrain, whether magical or nonmagical, that is made of plants, mud, swamp or similar effects. 

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Mask of the Wild. The dragon can use the hide action as a bonus action on each of its turns. It can attempt to hide even if it is only lightly obscured by rain, mist, leavage and other natural phenomena

Poison Absorbtion. When the dragon is subjected to poison damage, it takes no damage and instead regains an amount of hit points equal to the poison damage.

Spellcasting. The dragon is a 20th level spellcaster. It's spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks). It can cast the following spells. requiring no material components:

• Cantrips (at will): friends, mage hand, poison spray, prestidigitation, thaumaturgy
• 1st level (4 slots): detect magic, identify, dissonant whispers, silent image, shield
• 2nd level (3 slots): invisibility, misty step, suggestion
• 3rd level (3 slots): major image, sending, remove curse
• 4th level (3 slots): death ward, elemental bane, hallucinatory terrain
• 5th level (3 slots): mislead, summon draconic spirit, scrying
• 6th level (2 slots): mass suggestion, programmed illusion
• 7th level (2 slots): divine word, reverse gravity
• 8th level (1 slot): dominate monster, feeblemind
• 9th level (1 slot): time stop

Toxic Skin.  A creature takes 21 (6d6) poison damage and must make a DC 25 Constitution saving throw or become poisoned until the end of its next turn if it touches the dragon or starts its turn while in contact with the dragon, such as when it is grappled by it.

Treasure Sense. The dragon knows the location of all magic items and of accumulations of mundane treasure within 5 miles of it.

Multiattack. The dragon can use its World-Ending Presence, and if it was flying at the start of its turn, it can use its Crushing Landing. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 25 ft., one target. Hit: 22 (2d12 + 9) piercing damage plus 21 (6d6) poison damage and the dragon can grapple the target as part of that attack.

Claw. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 20 (2d10 + 9) slashing damage plus 21 (6d6) poison damage. If the target is a creature, it must succeed on a DC 25 strength saving throw or be flung up to 3d4 × 10 feet away from the dragon, landing prone.

Tail. Melee Weapon Attack: +17 to hit, reach 35 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage plus 21 (6d6) poison damage and the target must make a DC 25 Strength saving throw or be knocked prone.

World-Ending Presence. Each creature of the dragon's choice that is within 300 feet of the dragon and aware of it must succeed on a DC 21 Wisdom saving throw or drop whatever it was holding and become frightened for 1 minute. A frightened creature must use the dash action on each of its turns, moving as far away from the dragon as it can, using the safest possible route.  If a creature ends its turn in a loaction where it does not have line of sight to the dragon, it can repeat the saving throw, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's World-Ending Presence for the next 24 hours.

Crushing Landing. If the dragon is flying, it can attempt to land forcefully in a space of its choice, even if that space is occupied by other creatures. When it does so, its movement speed is reduced to 0 until the end of the turn, and each creature in the dragon's space or within 30 feet of it, other than the dragon itself, must make a DC 25 Dexterity saving throw or take 26 (4d12) bludgeoning damage plus 21 (6d6) poison damage and be knocked prone.

Poison Breath (Recharge 5–6). The dragon exhales fire in a 300-foot cone. Each creature in that area must make a DC 25 Constitution saving throw, taking 98 (28d6) poison damage on a failed save, or half as much damage on a successful one. In addition, on a failure a creature is poisoned until the end of its next turn.

Quicken Spell (3/day). The dragon casts one of its spells of 5th level or lower with a casting time of 1 action.

Act of Treachery.  When a creature that the dragon can see within 120 feet of it attempts to attack the dragon or to cast a spell at it, the dragon can force the creature to make a DC 21 Wisdom saving throw or be charmed and under the dragon's control until the end of this turn.

The dragon can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 30 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 19 (2d8 + 10) bludgeoning damage plus 21 (6d6) poison damage and be knocked prone. The dragon can then fly up to half its flying speed.

Cast a Spell (Costs 2 Actions). The dragon casts one of its spells of 5th level or lower.

Poison Mist (Costs 3 Actions). The dragon emits stinking, poisonus fumes within a 120-foot radius centered on itself. The area becomes heavily obscured until the end of the dragon's next turn and each creature within the area must make a DC 25 Constitution saving throw. On a failure, it takes 21 (6d6) poison damage, is  poisoned until the end of its next turn and spends its action on its next turn retching and reeling. On a success, a creature takes half as much damage without any additional effect.